The Spatio-Temporal Control of Zygotic Genome Activation

One of the earliest events in embryonic development is zygotic genome activation (ZGA). According to nascent transcript profiling most zygotic genes are inactive until the mid-blastula transition (MBT), and it has been suggested that only at this stage is the cell cycle slow enough, and the nuclear-to-cytoplamic (N/C) ratio of maternal repressors low enough, for bulk transcription to occur. Here we resolve the ZGA of the frog Xenopus tropicalis in time and space. We detect a gradual increase in quantity and length of RNA polymerase II-mediated elongation before the MBT, indicating that ZGA does not depend on a specific N/C ratio, and we observe that the size of newly transcribed genes is not necessarily constrained by cell cycle duration. We also reveal that canonical Wnt, Nodal and BMP signaling jointly generate most of the spatio-temporal dynamics of regional ZGA, converting the egg into a multi-axial embryo with proportionate germ layers.